The present invention relates in general to an electrochromic layer system, and in particular to a system where a layer or coat of an electrochromic material and at least one additional layer or coat having a matrix of a solid inorganic substance as the carrier of an electrolyte are arranged on a substrate between two electrodes, of which at least one is transparent.
Electrochromic layers are layers which change their absorption behavior under the action of an electric field, that is, their transmission color under the action of a field of a predetermined direction, or which return to their original state when the field disappears or is reversed. This phenomenon is based on the appearance of new or on the displacement of existing absorption bands in the materials. Of particular importance are those (so-called "long-lasting") electrochromic materials where the state brought about by the electrical field (e.g. a blue coloration) remains after the field is disconnected and can only be removed when an opposing field is applied. These materials are particularly suitable for displays, since the maintenance of certain display state does not require a permanent electric field, only corresponding voltage pulses are required to change the display.
The electrochromic materials are generally insulating or semi-conductive materials. The long-lasting electrochromic materials are mostly inorganic materials which contain polyvalent chemical elements which can form several oxidation states. Suitable are, e.g. the metallic transition metals, like copper. As electrochromic coating materials which can contain the said elements in several oxidation states are used, e.g. oxides, selenides, tellurides, chromates, molybdates, tungstenates and similar compounds. Particularly tungsten oxide is widely used as an electrochromic layer material, which passes over by an electric field from a state that appears colorless absorption free in the visible spectral range into a state in which it appears blue in transmitted light produced by a wide absorption band with a maximum at about 1.5 mu m. The thickness of electrochromic layers is preferably between 0.1 and 10 mu m, and even low voltages of a few Volt suffice already to produce a high field strength in the layer by means of applied electrodes.
In order to obtain a reversible electrochromic layer arrangement, it is necessary to apply an electrolytic layer between the electrodes in addition to an electrochromic layer, that is, a layer which changes in its optical transmission under the influence of an electric field. This electrolytic layer has the function, according to present thinking of injecting ions into the electrochromic layer with a corresponding field direction. Possible ion-sources are particularly strong acids, (e.g. sulfuric acid) and bases, but also plastics, which either contain themselves acid or basic groups, or which act as carriers of electrolytes with which they are impregnated. Frequently gels and pastes containing an electrolyte are also used in known arrangements.
The strong electrolytes have the great disadvantage that they are mostly aggressive and corrode the electrochromic metal and frequently also the electrodes, so that the useful life of these arrangements is considerably reduced. In order to eliminate this disadvantage as far as possible, it has been suggested to provide protective coats, e.g. of silicon oxide, calcium fluoride, magnesium fluoride, or similar insulating materials which can be arranged either only between the electrolytic layer and the electrochromic layer, or on both sides of the electrolyte. This also solved the problem of a good contact between the electrolytic layer and an adjacent transparent electrode, e.g. a gold electrode. With sufficient thickness of the protective coat it can thus be achieved that the electrically insulates the electrode, which seems necessary for the operation of the electrochromic device, but remains at the same time sufficiently permeable to ions. For silicon oxide as a protective coat is recommended, for example, a thickness of 3.4-4.5 mu m. In order to further increase the adhesion of gold electrodes on the protective coat, it has been suggested to provide so-called nucleation coats in the manufacture between the said two coats particularly of palladium, because it is known that such a germination of a substrate, before the following metal coat is applied, can greatly increase the bond between the two coats.
In order to avoid the difficulties encountered with strong acids or bases as electrolytes, as well as with the use of semi-liquid or pasty electrolytic layers or, inorganic layers have already been used for the electrolytic layers, where water molecules originating from the humidity of the air could accumulate on their surface or pores, so that obviously a sufficient amount of ions was formed, probably due to the foreign substances dissolved in the water. Preferably oxides and fluorides were used for the electrolytic layers. But this raised another problem: the layer systems had to remain open to the atmosphere, so that they could absorb water vapors from the environment, perhaps also carbon dioxide, ammonia and gases which contributed to the ion-formation. A disadvantage of these open systems was the great dependence of their functioning on the respective atmospheric conditions. When it was tried to seal the systems from the outside air, they soon failed, e.g. after 1000 switching cycles, while a life of at least 10.sup.6 switching cycles would be desirable. Obviously the water or other substances originating from the layer system, which are necessary for the electrolysis, are irreversibly used up in the operation of these electrochromic devices which is also suggested by the fact that the light absorbing products are frequently deposited in the course of time on the electrode adjoining the electrolytic layer, which is particularly annoying in the transparent electrode.
In order to suppress the formation of deposits, a film of oxidation reduction polymer, a filler polymer film or a layer in the form of a paste can be provided additionally between the electrolytic layer and the adjoining electrode according to DOS No. 28 54 812, which contains an oxidizing or reducing form of a substance. Particularly recommended for this purpose is a coat in the form of a filler-containing polymer film based on polyvinyl-alcohol with graphite powder as a filler, where the oxidized or reduced form of the substance forms a reversible oxidation-reduction electrode, where there is no storage of products of non-reversible electrochemical reactions. The object was to seal the system hermetically, but a disadvantage of this known arrangment, specifically for transparent systems, is that this additional coat (particularly when it contains graphite powder as a filler), is itself interfering by its light absorption.